Retroreflective materials are used extensively in making highway signs, street signs, pavement markers, and the like. A material which is retroreflective is characterized by its ability to return light toward the source of a non-perpendicular incident ray. This type of reflection is to be distinguished from mirrors, which cause specular reflection, or from diffuse reflecting surfaces, which scatter the incident light in all directions.
Retroreflective materials which employ cube-corner type reflecting elements are now well-known in the art. Cube-corner reflecting elements are trihedral structures which have three mutually perpendicular lateral faces meeting at a single corner, such as that configuration defined by the corner of a room or a cube. The retroreflectivity typically achieved by cube-corner type reflecting elements is through the principle of total internal reflection. A transparent cube-corner element receives a ray of incident light at an angle and sends it back in the same direction. See, for example, U.S. Pat. Nos. 3,924,929 and 4,672,089.
Cube-corner type retroreflective materials typically exhibit progressively less reflectivity as the incident angle of viewing light is increased. Generally, such materials lose significant amounts of reflectivity at incident angles greater than about 20.degree., and lose nearly all of their reflectivity when the incident angle becomes greater than about 40.degree.. The angle of incidence is measured from the line perpendicular to the surface of the retroflective sheeting material.
A retroreflective material which utilizes the retroflective properties of a cube-corner, but which remains efficient at very high angles of incidence is described in the White patent, U.S. Pat. No. 4,349,598 (hereinafter "White" patent). The retroreflective material taught in the White patent is able to obtain retroreflection from a surface which is nearly parallel to the incident light. The reflecting elements described in the White patent are reflecting, transparent, right triangle prisms having a rectangular base, two mutually perpendicular rectangular faces meeting the base at 45.degree. angles, and two parallel triangular faces perpendicular to the rectangular faces.
While successful in solving the problems of the use of retroreflective material at high incident angles, the White construction has certain drawbacks. The required vertical cuts between the White reflecting elements cause a 25 to 50% loss of the effective area of the material from which light is retroreflected (effective area). Also, the presence of the two parallel vertical surfaces at each end of the individual reflective elements makes molding a difficult task. The vertical surfaces provide no relief angle to aid in removal of the reflective sheeting from the mold. As a result, great stress is exerted on each of the individual retroreflective elements as the material is stripped from the mold. Such stresses can lead to deformation of the individual retroreflective elements, causing deviations in the intended retroreflection pattern of the retroreflective material.
Many cube corner designs, including the White design, are not designed to obtain a maximum effective aperture for each retroreflective element. For example, the individual elements of a canted trihedral cube corner material may have an effective aperture of roughly 30%, and an element of the White design roughly 70%.
Therefore, a need exists for an improved retroreflective material which is effective at high angles of incidence. Further, there is a need for such a retroreflective material which has elements with a high percentage of effective aperture, a high percentage of effective area of the sheeting, and which has a design which is easily molded in commercial molding techniques.